EcoRenovator - View Single Post

superlen · 11-19-15, 11:29 AM

Since I stumbled onto your thread first (Edit errr second.

- Tech posted in while I was typing), I'll post my thoughts on this. Warning, I'm not a refrigeration guru by any means, but I have done enough research to toss about ideas like I know what I'm doing.... I don't. Anyone feel free to correct me.

I too, like most, have pondered the effects of changing the size of your heat exchangers (HEXs - evap or cond) . If you spent any time on hvac-balk you'll know that you can never ever ever change the size of either HEX on any system ever. Why would you? what are you a dumbass? we all know they have to be matched...and installed by a licensed hvac contractor.. you shouldn't even be asking this question, only cheapskate diy guys wonder this.. you should only touch the thermostat after a licensed hvac tech has approved you. Of course by the fact that we are in this site, we know all that is BS.

Having made fun of that position, there obviously is a reason you can't just change one major piece of the puzzle and expect it will work perfect. It might, might not. In your case, if you changed both HEX from air to water & somehow managed to keep the amount of heat rejected/accepted approximately the same as what the original system was designed for, your stock cap tube should be sized appropriately. It's when you drastically change the heat transfer rate on the HEX is when you now have to consider what the ramifications are.

A cap tube or TXV both do the same thing. They are the metering device (MD) that sets the flow rate of refrigerant through the system. More flow, more heat moved, less flow less heat moved. In a cap tube system it is sized for a certain amount of BTU transfer of both the evap and the cond. It shouldn't matter what the physical evap or cond is, water, air, cooling tower, chunck of earth, solar panel, ect., as long as that HEX moves the BTU Rate that the MD is expecting things should be ok. Note: It's not the size of the HEX, its how many BTUS/HR it can move. A 4t 10seer AC from 1990 has an outside condenser size of X & a big powerful fan to move a lot of air across it. The same 4T 16 seer system from 2015 has a much larger condenser, so the fan can move less air. The same amount of BTUS/HR are moved with both condensors, yet the 16seer with it's smaller fan uses less total power, hence the efficiency boost. In theory you can have an air condenser that is MASSIVE and convection alone would move enough heat (no fan - very efficient... a DX system operates similar to this with earth conduction). In practice other variables with this fanless system cause issues, sheer size, mnf cost, oil transport, more friction of refrigerant over more lines, others I haven't thought of. I still am wanting to play with this exact concept on my home AC (use existing condenser coil to dump as much heat as possible by natural convection - no fan - then let a second water condenser take up the remaining btu rejection required).

So back to your question and system, I think the way to approach it is like this:
1. Start with evaporator and figure out if you're new evaporator is going to move more heat or less heat than the original. If more, you will need more flow rate (bigger diameter, shorter tube). If less, the opposite.

2. Make your condenser design so that it can move the same amount of heat (or more) than your evaporator.

3. Start experimenting.. I think the above two statements will keep you close to a working systems. Others, with more real world experience than I should chime in.

Your setup is going to be a little more difficult to predict as you have two tanks of water that start out a certain temp. There is no flowing water in this case so the rate of heat movement is not fixed. It will start at one rate and as the water heats/cools the rate of heat movement will decrease over time which of course would change what size MD you need.

Another note as for using too large a TXV. It will hunt around if it is over sized, modulating from fully open to fully closed. A TXV is more forgiving of load changes and mis-matched HEX than a cap tube is, but even still it needs to be somewhat close. I *think* on larger systems 3,4,5T you can go up or down 1T and still be in range. On a smaller system, this would of course shrink. The TXV valve datasheet will list details that should help.

Hope some of this was more helpful than confusing, and again anyone feel free to point out errors in my logic.

Len

11-19-15, 11:29 AM	#4
superlen Helper EcoRenovator Join Date: Nov 2015 Location: NW Arkansas Posts: 37 Thanks: 5 Thanked 7 Times in 7 Posts	Since I stumbled onto your thread first (Edit errr second. - Tech posted in while I was typing), I'll post my thoughts on this. Warning, I'm not a refrigeration guru by any means, but I have done enough research to toss about ideas like I know what I'm doing.... I don't. Anyone feel free to correct me. I too, like most, have pondered the effects of changing the size of your heat exchangers (HEXs - evap or cond) . If you spent any time on hvac-balk you'll know that you can never ever ever change the size of either HEX on any system ever. Why would you? what are you a dumbass? we all know they have to be matched...and installed by a licensed hvac contractor.. you shouldn't even be asking this question, only cheapskate diy guys wonder this.. you should only touch the thermostat after a licensed hvac tech has approved you. Of course by the fact that we are in this site, we know all that is BS. Having made fun of that position, there obviously is a reason you can't just change one major piece of the puzzle and expect it will work perfect. It might, might not. In your case, if you changed both HEX from air to water & somehow managed to keep the amount of heat rejected/accepted approximately the same as what the original system was designed for, your stock cap tube should be sized appropriately. It's when you drastically change the heat transfer rate on the HEX is when you now have to consider what the ramifications are. A cap tube or TXV both do the same thing. They are the metering device (MD) that sets the flow rate of refrigerant through the system. More flow, more heat moved, less flow less heat moved. In a cap tube system it is sized for a certain amount of BTU transfer of both the evap and the cond. It shouldn't matter what the physical evap or cond is, water, air, cooling tower, chunck of earth, solar panel, ect., as long as that HEX moves the BTU Rate that the MD is expecting things should be ok. Note: It's not the size of the HEX, its how many BTUS/HR it can move. A 4t 10seer AC from 1990 has an outside condenser size of X & a big powerful fan to move a lot of air across it. The same 4T 16 seer system from 2015 has a much larger condenser, so the fan can move less air. The same amount of BTUS/HR are moved with both condensors, yet the 16seer with it's smaller fan uses less total power, hence the efficiency boost. In theory you can have an air condenser that is MASSIVE and convection alone would move enough heat (no fan - very efficient... a DX system operates similar to this with earth conduction). In practice other variables with this fanless system cause issues, sheer size, mnf cost, oil transport, more friction of refrigerant over more lines, others I haven't thought of. I still am wanting to play with this exact concept on my home AC (use existing condenser coil to dump as much heat as possible by natural convection - no fan - then let a second water condenser take up the remaining btu rejection required). So back to your question and system, I think the way to approach it is like this: 1. Start with evaporator and figure out if you're new evaporator is going to move more heat or less heat than the original. If more, you will need more flow rate (bigger diameter, shorter tube). If less, the opposite. 2. Make your condenser design so that it can move the same amount of heat (or more) than your evaporator. 3. Start experimenting.. I think the above two statements will keep you close to a working systems. Others, with more real world experience than I should chime in. Your setup is going to be a little more difficult to predict as you have two tanks of water that start out a certain temp. There is no flowing water in this case so the rate of heat movement is not fixed. It will start at one rate and as the water heats/cools the rate of heat movement will decrease over time which of course would change what size MD you need. Another note as for using too large a TXV. It will hunt around if it is over sized, modulating from fully open to fully closed. A TXV is more forgiving of load changes and mis-matched HEX than a cap tube is, but even still it needs to be somewhat close. I think on larger systems 3,4,5T you can go up or down 1T and still be in range. On a smaller system, this would of course shrink. The TXV valve datasheet will list details that should help. Hope some of this was more helpful than confusing, and again anyone feel free to point out errors in my logic. Len Last edited by superlen; 11-19-15 at 12:22 PM..